1. Field of the Invention
This invention generally relates to a ball joint assembly, which is also called a ball-and-socket joint assembly, and, in particular, to a ball unit for use in a ball joint assembly including a ball unit and a socket unit operatively coupled to the ball unit.
2. Description of the Prior Art
A ball joint assembly or ball-and-socket joint assembly is well known in the art, and it generally includes a ball unit having a ball portion and a socket unit having a socket portion in which the ball portion of the ball unit is received. In such a ball joint assembly, the ball unit may move relative to the socket unit within a certain angle in all planes passing through a center line defined by the socket portion of the socket unit. The prior art ball unit for use in such a ball joint assembly generally includes three sections, i.e., (1) a ball portion, (2) a nut (tightening) section, and (3) a threaded or holding section, arranged in the order mentioned along a longitudinal center line of the ball unit from one end to the other. In addition, a cut-away section is typically provided in the form of a circumferential groove between the ball and nut sections for manufacturing reasons.
In accordance with a typical prior art method of manufacturing a ball unit of a ball joint assembly, a columnar member is first processed by cold heading to thereby form an intermediate product which has an outer shape substantially close to a finished product. Then, only the ball portion of the intermediate product is processed by roll rolling at high precision, so that there is obtained a second intermediate product which has an outer shape very similar to that shown in FIG. 3. Thereafter, a cut-away portion is provided between the ball and nut sections in the form of a circumferential groove and then the intermediate product is also processed by thread rolling to thereby form a threaded section to complete the manufacture of the ball unit.
In a cold heading process, a columnar member of raw material is fed into a cavity defined between a pair of top and bottom mold halves in contact, defining a predetermined shape of its inner surface. As a result, the shape of the mold cavity must be made smaller gradually deeper into the cavity along its longitudinal axis, otherwise the product thus processed by cold heading cannot be removed from the cavity. In the present case, since the center of the ball portion of a ball unit to be manufactured is defined as a split plane between the two mold halves, the ball unit must have a structure whose outer shape becomes gradually smaller from its ball portion toward its threaded portion along the longitudinal axis of the ball unit (see FIG. 3). If it is desired to extend the ball section beyond the diameter which is defined by the maximum diameter defined by the nut section which is adjacent to the ball section so as to provide an increased spherical surface, the extended portion of the ball section is defined by a cutting process at the time when the cut-away section is formed between the ball and nut sections in the form of a circumferential groove. Accordingly, the extended portion naturally has a decreased precision as compared with the remaining portion of the ball section, so that the overall degree of sphericity tends to be reduced.
On the other hand, in the absence of such an extended portion, e.g., the ball section terminating in a cylindrical portion having the diameter D which is the diameter of the largest circle definable by the nut section, the ball section presents only a limited spherical surface. Thus, when such a ball unit is combined with a socket unit by placing the ball section within the socket section of the socket unit, the contact surface between the ball and socket sections is limited, which tends to decrease the amount of load which can be borne between the ball and socket units.